The present invention relates to an optically transparent film which transmits visible light, and more particularly to an optically transparent film containing amorphous fluorocarbon, a method of manufacturing the same, an alignment film including the optically transparent film, and a liquid crystal panel and a display device using the alignment film.
Display devices, liquid crystal display devices in particular, have been widely used in recent years, and the scope of their application is increasing. A liquid crystal display device can be configured in the following manner: an electrode is formed of a transparent electrode material such as ITO (Indium Tin Oxide) on each of opposite transparent substrates made, in general, of glass, a liquid crystal material is filled into a gap between the substrates each having the electrode formed thereon, and the liquid crystal material is sealed in between the substrates by means of a sealing material. An alignment film generally made of a material such as a polyimide film or a carbon film is formed on a surface, facing the liquid crystal material, of the electrode formed on each substrate, and the alignment film is adapted to align the liquid crystal near the substrate so as to impart necessary optical properties to the liquid crystal.
A description will now be given with regard to the manufacture of the above-mentioned alignment film. When an organic film such as polyimide is used for manufacturing the alignment film, a polyimide precursor such as polyamic acid is coated on a transparent electrode. The polyamic acid undergoes pre-baking and further undergoes post-baking to transform the polyamic acid into a polyimide film. The polyimide film formed in this manner is not adequate to align liquid crystal, and therefore, rubbing takes place to subject the polyimide film to the process for aligning the liquid crystal.
On the other hand, when an inorganic material such as a carbon film is used to form the alignment film, the carbon film is formed on a transparent electrode by a method such as vapor deposition, CVD (chemical vapor deposition) or sputtering, and thereafter the carbon film is subjected to an alignment process by irradiation with a particle beam such as an ion beam, an atomic beam or a molecular beam in order to impart sufficient alignment to the carbon film. Although the above-described processes allows conventional alignment films to provide sufficient properties, a hydrogenated amorphous carbon film is often useful as the carbon film, that is, a so-called a-C:H film, for example. An a-C:H film can have a relatively low transmittance in the visible light range. To apply the a-C:H film to an optically transparent film which transmits visible light, the film thickness thereof must be therefore minimized to be about 10 nm or less, and this results in considerable limitations during the deposition process.
In addition, the relative dielectric constant of the a-C:H film is from about 4.8 to about 6.1 inclusive in the frequency range of 100 Hz to 10 kHz, which is obtained by actual measurement on the a-C:H film having a film thickness of about 300 nanometers (nm), and the relative dielectric constant thereof is known to lie in the range of about 4.5 to 6.5 even after allowing for the conditions of measurement and the properties of samples. An a-C:H film can have a dielectric constant largely different from the dielectric constant of the layer being located adjacent to the a-C:H film. Therefore, an electric charge can be stored on the interface between the a-C:H film and the adjacent layer, and the stored charge results in so-called space charge, which consequently forms a potential barrier.
When the above-mentioned space charge is accumulated, for example, in a liquid crystal display device, an electric field in the liquid crystal remains even after the removal of the driving electric field, and this causes electrical or electronic trouble in particular, such as a trouble that image-sticking is perceptible due to the remaining electric field. An optically transparent film, which is sufficiently optically transparent, has a low dielectric constant and can be easily manufactured, has been heretofore required.
Various attempts have been heretofore made to solve the above-mentioned problems. For example, a coating film including a thin coating film made of graphite fluoride is disclosed in Examined Japanese Patent Application Publication No. Sho (1977)52-42388. The graphite fluoride coating film is provided with crystal structures so as to be used as the alignment film of the liquid crystal display device. Although the coating film of graphite fluoride formed in this manner exhibits excellent properties to function as the alignment film, the coating film has low transmittance in visible light because the coating film is crystallized. Thus, the coating film must be formed with a thickness of less than about 10 nm in order to impart sufficient optical properties to the coating film, and this results in strict limitations on the manufacturing process.
Moreover, a coating film made of pitch fluoride is applied to the alignment film for use in the liquid crystal display device in Unexamined Japanese Patent Application Publication No. Hei 4(1991)-204827. Although a coating film of pitch fluoride is adequate to align liquid crystal molecules, the coating film has poor optical properties because it includes crystal structures, and thus the coating film is strictly limited in the manufacturing process in the same manner as the conventional a-C:H film. Therefore, the coating film of the pitch fluoride disclosed in Unexamined Japanese Patent Application Publication No. Hei 4(1991)-204827 has the disadvantage of increasing the costs of the manufacturing process, although the coating film of pitch fluoride has the advantage of reducing material costs.
The present invention is directed at overcoming the problems set forth above.
It is an object of the present invention to provide optically transparent film and a method of making the film having sufficient optical transparency and a low dielectric constant that can be easily manufactured.
It is another object of the present invention to provide an alignment film for aligning a liquid crystal material, which is formed from the optically transparent film of the present invention.
It is still another object of the present invention to provide a liquid crystal panel and a display device, which use the alignment film formed from the above-mentioned optically transparent film.
According to one aspect of the invention, there is provided an optically transparent film comprising an amorphous fluorocarbon compound.
According to another aspect of the invention, there is provided a method of manufacturing an optically transparent film, the method comprising the steps of providing a substrate, exposing the substrate to an atmosphere containing carbon to form the optically transparent film having an integrated transmittance on the substrate, and exposing the substrate to an atmosphere containing hydrogen and fluorine to include fluorine in the optically transparent film to the extent that the content of fluorine in the optically transparent film controls the integrated transmittance of the optically transparent film within the visible region.
According to yet another aspect of the invention, there is provided a panel comprising a first substrate having an external surface, a first optically transparent film including an amorphous fluorocarbon compound positioned on the external surface of the first substrate, and a liquid crystal layer positioned on the first optically transparent film.
For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings.